As bandwidth requirements of communication networks continue to rise, wavelength division multiplexing is being used increasingly to aggregate the traffic of many users onto the optical fiber of backbone networks. For example, using a wavelength division multiplexer (WDM), Eighty or more separate wavelengths or channels of data can be multiplexed into a light signal transmitted on a single optical fiber. If each channel carries 2.5 Gbps (billion bits per second), up to 200 billion bits per second can be delivered on the single optical fiber.
However, in wavelength division multiplexing systems, the signal power levels transmitted in an optical fiber depend on the wavelengths. These inter-wavelength discrepancies in optical power levels are caused in part by the use of optical amplifiers, such as erbium-doped fiber amplifiers (EDFAs). The use of EDFAs has revolutionized fiber optics, as they enable WDM data transport over thousands of kilometers. Unfortunately, as EDFAs do not inherently have a flat gain spectrum, there is the problem of uneven gain for different wavelengths. Variable Optical Attenuators (VOAs) provide a solution to this problem by attenuating different wavelengths by different amounts, therefore flattening the gain spectrum.
FIG. 1 is a schematic of an exemplary prior aft application of a variable optical attenuator (VOA). A multiple number of VOAs, e.g., 110-1, 110-2, and 110-3, each receive an input wavelength, e.g., λ1, λ2, and λn, respectively. The VOAs attenuate the power of each input wavelength by different amounts and then transmit the attenuated wavelengths to the WDM 112 to be multiplexed into a multi wavelength light signal. This multi wavelength light signal is the amplified by an EDFA optical amplifier 114 and output to a fiber optic cable for transmission. The attenuation for each VOA has been chosen to compensate for the uneven gain spectrum of the optical amplifier 114.
VOAs in current use include either Mach-Zender interferometers which use a thermo-optic effect to cause variation in attenuation or an electronically controlled mechanical means to cause variation in attenuation. One of the significant disadvantages of these typical VOAs is the speed (i.e., long settling or slow response times). Hence for the fast optical switching networks, which need high speed power adjustments on the order of about one nanosecond (1 GHz), current VOAs are inadequate. Therefore what is needed is a VOA with high speed attenuation adjustment that can support fast optical switching networks.